The perceived brightness of LEDs can be dimmed by using a PWM dimmer to control the duty cycle of constant-magnitude current pulses to the LEDs. The PWM frequency is above the frequency that flicker can be perceived, such as above 60 Hz and commonly 100-1 KHz. The ideal current per pulse is that current which provides optimal operation of the LEDs.
The LED's peak wavelength and efficiency change with driving current. For example, with GaN LEDs, there is a blue shift as the current increases. Therefore, it is desirable for the current per pulse to be a constant target current throughout the entire pulse. The current is typically supplied by a switching regulator that switches at a frequency (e.g., >100 kHz) much higher than the PWM dimming frequency, so many regulator switching pulses occur during a single PWM dimming ON-time. Such high frequency pulses are smoothed by an output capacitor so the current through the LEDs is essentially a direct current for the duration of the pulse.
The switching regulator receives feedback from the sensed LED current to generate the target current during an ON-time of the PWM dimming pulse. Basically, when the PWM dimming controller generates an ON pulse, the regulator starts up to generate the target current and, simultaneously, the LED load is connected to draw current from the regulator. When the PWM dimming controller terminates the ON pulse, the regulator stops operating, and the LED load is either disconnected from ground or disconnected from the regulator to stop drawing current from the regulator's output capacitor. During the OFF-time, the regulator's inductor current typically drops to zero.
One problem with such a typical PWM dimming system is that, upon the rising edge of the PWM dimming ON pulse, when the regulator starts up and the LED load begins to draw current, there is an immediate surge of current into the LED load from the output capacitor. This reduction of charge in the output capacitor, without the regulator having time to replenish the charge, rapidly lowers the output current of the regulator, and, in turn, has an effect on the feedback (the sensed LED current) to the regulator. Thus there is a transient, and some time is taken in order for the regulator to achieve a steady state current output to the LEDs during the current pulse. This initial current ramp-up delay into the LEDs is due to the current ramping through the regulator's inductor starting from zero current at the onset of a PWM dimmer ON-time. The regular may take many switching cycles, such as greater than 20, until the current delivered from the inductor equals the target direct current through the LEDs (i.e., until steady-state is reached). The LED's perceived brightness (and to a lesser degree the peak wavelength) is determined by both the magnitude of the direct current supplied by the regulator during the PWM ON-time and the duty cycle of the ON pulses. Therefore, it is important that the regulator achieve steady-state as soon as possible after the PWM ON pulse is asserted.
The number of switching cycles needed by the regulator to achieve the target direct current depends on the voltage across the inductor (e.g., the difference between the input voltage and the output voltage in a buck regulator), the inductor value, the starting current through the inductor, the target direct current, the switching frequency, and other factors. Since the regulator requires a number of switching cycles (e.g., greater than 20 cycles) before reaching the steady state direct current, the minimum ON-time of the PWM ON pulse is limited. A human should not perceive any color shift with different PWM dimming duty cycles, so the minimum ON-time should be sufficiently long to cause the LEDs to generate the same peak wavelength during a vast majority of the PWM dimmer ON-time. This may limit the minimum ON-time to a 10% duty cycle or more. As an example, if a 100 KHz switching regulator takes 20 cycles (2×10E-4 sec.) to ramp up the inductor current to the target inductor current, this time is equal to the ON-time of a 2% duty cycle in a 100 Hz PWM dimmer. Therefore, with the 2% duty cycle, the smoothed current pulse to the LEDs is less than the target LED current. Since the human perception of brightness is logarithmic, the 2% minimum duty cycle is significant if the user desires a very dim light. Additionally, since the brightness and impedance of LEDs (being forward biased diodes) is also not a linear function of the current, any current less than the target direct current may cause the LEDs to not illuminate at all, resulting in the LEDs being completely off at duty cycles less than 2%.
One solution to this problem is described in U.S. Pat. No. 8,294,388, which is intended to reduce audible noise from the regulator's output capacitor when the LED load initially draws current from the capacitor when the PWM dimmer ON pulse is generated. The circuitry described in that patent senses the PWM ON pulse and delays applying it to the regulator. Upon sensing the PWM ON pulse, the system pre-charges the inductor to some current level that has been calculated based upon the input voltage, the output voltage, and the inductor value. After the desired pre-charge current level is reached, the regulator is controlled to begin operating and the LED load draws current for the remainder of the PWM dimmer ON-time. However, during the delay time when the inductor is pre-charging up from a low current level, the output capacitor is discharging into the inductor, so its voltage is lower than the voltage needed to supply the target current to the LED load. When the inductor current reaches its target pre-charge level, the regulator begins to operate and the LED load begins to draw current. However, at this time, the capacitor voltage is still below the steady-state level it was at when the previous PWM dimmer ON-time ended, resulting in the current to the LED load to be initially low. Thus, the regulator's feedback loop must correct for the initial drop in output voltage, and the regulator must then take multiple switching cycles to achieve steady state. Due to the added time it takes for the prior art regulator to achieve steady-state, the minimum PWM duty cycle achievable is still limited, and the user cannot fully control the dimming of the LED brightness down to low levels.
Additionally, the prior art system must calculate the optimal pre-charge current level of the inductor using the measured input voltage and output voltage, and other parameters. This precision measurement becomes complex, and the optimal pre-charge level will not be achieved, especially in a boost converter.
Additionally, the prior art patent does not mention delaying the dimmer ON pulse falling edge (applied to the regulator) by the same amount the ON pulse rising edge was delayed. This is especially problematic at very low duty cycles when the ON pulse is short, since the dimming becomes progressively non-proportional.
Additionally, by delaying the dimmer ON-pulse, synchronization of the regulator operation with other circuitry using the original PWM dimmer signal is much more difficult, or not possible.
What is needed is a technique to control the dimming of LEDs driven by a switching regulator that allows the PWM minimum duty cycle to approach zero to maximize the dimming ratio, while a substantially constant current is supplied to the LED load throughout the entire ON-time.